1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using a photoreceptor, and to an electrophotographic image forming method. In addition, the present invention also relates to a process cartridge for use in an electrophotographic image forming apparatus.
2. Discussion of the Background
Electrophotographic image forming methods are widely used for copiers, facsimile machines, laser printers, direct digital printing plate making machines, etc. The electrophotographic image forming methods typically include the following processes:
(1) charging a photoreceptor (charging process);
(2) irradiating the photoreceptor with imagewise light to form an electrostatic latent image thereon (imagewise light irradiation process);
(3) developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor (developing process);
(4) transferring the toner image onto a receiving material such as paper optionally via an intermediate transfer medium (transfer process);
(5) fixing the toner image on the receiving material, for example, upon application of heat and pressure thereto (fixing process); and
(6) cleaning the surface of the photoreceptor (cleaning process).
Recently, copiers, facsimile machines and laser printers tend to be personalized. Therefore, such image forming apparatus need to have high durability and stability (i.e., being maintenance-free) and to be small in size.
In addition, such image forming apparatus need to stably produce high quality images because recently the performance of image scanners and personal computers from which images are input to the image forming apparatus has been dramatically improved.
As the electrophotographic photoreceptor (hereinafter referred to as a photoreceptor) for use in such image forming apparatus, inorganic photoreceptors using a material such as selenium or amorphous silicon and organic photoreceptors are known. Among these photoreceptors, organic photoreceptors have been typically used because of having low costs, good designing flexibility and being non-polluting.
Specific examples of the organic photoreceptors include photoreceptors having the following photosensitive layers:
(1) photosensitive layers including a photoconductive resin typified by polyvinylcarbazole (PVK);
(2) photosensitive layers including a charge transfer complex typified by polyvinylcarbazole-2,4,7-trinitrofluorenon (PVK-TNF);
(3) photosensitive layers including a pigment dispersion typified by a phthalocyanine-binder system; and
(4) functionally-separated photosensitive layers using a combination of a charge generation material and a charge transport material.
Among these photoreceptors, the functionally-separated photoreceptors attract attention.
The mechanism of forming an electrostatic latent image on a functionally-separated photoreceptor is as follows:
(1) when imagewise light irradiates a charged photoreceptor, the imagewise light is absorbed by a charge generation material in a charge generation layer after passing through a transparent charge transport layer located overlying the charge generation layer;
(2) the charge generation material absorbing light generates a charge carrier;
(3) the charge carrier is injected into the charge transport layer and transported through the charge transport layer (or the photosensitive layer) along an electric field generated by the charge formed on the surface of the photoreceptor; and
(4) the charge carrier neutralizes the charge on the surface of the photoreceptor, resulting in formation of an electrostatic latent image.
In the functionally-separated photoreceptors, a combination of a charge transport material having an absorption in an ultraviolet region and a charge generation material having an absorption in a visible region is known. In addition, recently laser diodes emitting light having a relatively short wavelength have been developed and used, and therefore various constructions have been investigated for the functionally-separated photoreceptors.
Various organic photoreceptors have been developed. However, in order that the photoreceptors are practically used, the photoreceptors need to have good electrophotographic properties such as high sensitivity, high potential, high potential retainability, good potential stability, low residual potential and proper spectral properties; good mechanical durability such as high abrasion resistance; good chemical stability against heat, light and discharge-induced products (e.g. ozone and NOx), etc.
In particular, a need exists for a photoreceptor having a small diameter because downsizing of electrophotographic systems is strongly desired. Therefore, a need exists for a photoreceptor having good resistance to abrasion which increases in proportion to the number of produced copies.
Thus, mechanical durability which typically means abrasion resistance is strongly needed. However, conventional organic photoreceptors and electrophotographic apparatus using the photoreceptors do not have high durability because the organic materials used have low abrasion resistance. In addition, the need for a photoreceptor having good abrasion resistance has increased because the thickness of a photosensitive layer has to be decreased to produce high definition images. Thus the designing flexibility of the photosensitive layers becomes smaller and smaller.
The reason why the thickness of a photosensitive layer has a particularly big influence on producing high definition images is considered as follows.
Among positive and negative carriers formed in the charge generation layer of a negatively chargeable multilayer organic photoreceptor by light irradiation, the negative carrier (i.e., an electron) is absorbed in the substrate and the positive carrier (i.e., a hole) is transported through the charge transport layer to the surface of the photoreceptor to be re-combined with the electron thereat, resulting in pair-disappearance of the hole and electron.
Due to this pair-disappearance, the electric field moving the hole toward the surface of the photoreceptor gradually decreases, and the hole moves toward a non-lighted area.
This is called a carrier scattering phenomenon in a direction of the surface of the photoreceptor, and because of the carrier scatter phenomenon, formation of a latent image faithful to the irradiated light is prevented. This results in formation of a poor image having low resolution.
The thickness of the charge transport layer has a big influence on the carrier scattering phenomenon, and making the thickness smaller is very effective for producing high resolution images.
In addition, laser irradiation, which is typically used recently, is different from irradiation using a halogen lamp, etc. because the incident photon speed of a laser is about 107 times that of a halogen lamp. Therefore, the density of the formed carriers is extremely high, and the electric field strength of the charge generation layer decreases because charges flow into the charge transport layer. Thereby, the carrier transport speed is decreased, resulting in late arrival of the carrier, which is formed by irradiation of the center portion of the laser beam, at the surface of the photoreceptor. The thus formed space charge distribution tends to cause carrier scattering in a direction parallel to the surface of the photoreceptor, resulting in deterioration of the image resolution.
In attempting to improve the abrasion resistance of an organic photoreceptor, Japanese Laid-Open Patent Publication No. (hereinafter referred to as JOP) 57-30846 discloses a photoreceptor in which a protective layer including a filler such as a metal or a metal oxide is formed on the surface of the photoreceptor. The object of this method is to increase the transparency of the protective layer to prevent increase of residual potential by using a filler having an average particle diameter not greater than 0.3 xcexcm. In addition, JOP 4-281461 discloses a photoreceptor, in which a charge transport material is included in a protective layer together with a filler to prevent increase of residual potential while maintaining good abrasion resistance. JOPs 53-133444 and 55-157748 have disclosed a photoreceptor, in which an organic acid is included in a protective layer together with a filler. JOP 2-4275 discloses a photoreceptor, in which an electron accepting material is included in a protective layer to prevent increase of residual potential.
The abrasion resistance may be improved by these techniques. However, the following problems tend to occur:
(1) a toner film is formed on the carrier used and/or the surface of the photoreceptor used and various members such as developing sleeves and cleaning blades; and/or
(2) the cleaning blade used is unevenly abraded due to asperities of the surface of the photoreceptor caused by the filler included in the photoreceptor, resulting in insufficient cleaning of the photoreceptor, and thereby background fouling is formed.
Thus, photoreceptors having good abrasion resistance while producing good images have not yet been developed.
JOPs 6-130711 and 2000-250245 have disclosed a photoreceptor which includes a fluorine-containing particulate resin and which has a specified surface roughness. In addition, the hardness and friction coefficient of the surface of the photoreceptor are specified therein. It is described therein that the photoreceptor has good cleaning property.
JOPs 8-262756, 9-190125, 9-288372 and 2001-265040 have disclosed photoreceptors in which an inorganic filler is included in an outermost layer and which have a specific surface roughness. In addition, these background art has disclosed the following techniques:
(1) a release agent is included in the toner:
(2) a cleaning blade is vibrated; and
(3) a fatty acid is supplied to the surface of the photoreceptor used.
However, according to the present inventors"" investigation, the characteristics of these photoreceptors deteriorate when the photoreceptors are used for a long period of time and/or environmental conditions are changed. Specifically, when the photoreceptors are run to produce not less than 200,000 copies, the photoreceptors become unstable, namely the durability of the photoreceptors is not satisfactory. It is found that maintenance of the surface condition of the protective layer is very important for forming high quality images.
Because of these reasons, a need exists for an electrophotographic image forming method and apparatus by which high quality images can be stably produced for a long period of time.
Accordingly, an object of the present invention is to provide an electrophotographic image forming apparatus which is more durable and stable than conventional electrophotographic image forming apparatus and which can produce images at a high speed.
Another object of the present invention is to provide a durable digital electrophotographic image forming apparatus in which high resolution images are written using a laser beam without causing undesired images such as tailing.
Yet another object of the present invention is to provide a process cartridge for use in an electrophotographic image forming apparatus, which is more durable and stable than conventional process cartridges.
A further object of the present invention is to provide an electrophotographic image forming method by which high quality images can be stably produced for a long period of time.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image forming apparatus including a photoreceptor serving as an image bearer which rotates in a direction and which includes an electroconductive substrate, a photosensitive layer located overlying the substrate, and a protective layer located overlying the photosensitive layer and including a filler; a latent image former configured to form an electrostatic latent image on a surface of the photoreceptor; an image developer configured to develop the electrostatic latent image with a developer including a toner to form a toner image on the surface of the photoreceptor; an image transfer device configured to transfer the toner image onto a receiving material optionally via an intermediate transfer medium; a cleaner configured to remove the toner remaining on the surface of the photoreceptor while contacting the surface of the image bearer; and a surface condition controller configured to control a surface condition of the photoreceptor.
The surface condition controller controls the surface condition of the photoreceptor such that the surface of the photoreceptor maintains a maximum height not greater than 3 xcexcm in a part of a profile of the surface of the photoreceptor, in a sampling range of 50 xcexcm in a direction perpendicular to the rotating direction of the photoreceptor.
The surface condition controller preferably includes a lubricant applicator configured to apply a lubricant to the surface of the photoreceptor. The lubricant applicator is preferably located at a position after the cleaner and before the image developer relative to the rotating direction of the photoreceptor.
The lubricant may be applied to the photoreceptor by a charger of the latent image former, a transfer belt of the image transfer device or a cleaning brush of the cleaner.
Alternatively, the lubricant may be applied to the surface of the photoreceptor by the toner which includes the lubricant. The toner may include a lubricant powder and toner particles. Alternatively the toner may include a wax as a lubricant.
The friction coefficient of the surface of the photoreceptor against paper is from 0.3 to 0.5 when measured based on an Euler belt method.
The latent image former preferably has a charger which charges the photoreceptor while contacting the surface of the photoreceptor (i.e., contact charging) or forming a narrow gap between the surface of the photoreceptor and the charger (i.e., proximity charging).
The charger preferably applies a DC voltage overlapped with an AC voltage to the photoreceptor.
Another aspect of the present invention, a process cartridge is provided which includes a photoreceptor and at least one of the latent image former, image developer, transfer device and cleaner mentioned above, wherein the surface of the photoreceptor maintains a maximum height not greater than 3 xcexcm in a part of a profile of the surface of the photoreceptor, in a sampling range of 50 xcexcm in a direction perpendicular to the rotating direction of the photoreceptor, and/or the surface of the photoreceptor has a friction coefficient of from 0.3 to 0.5.
Yet another aspect of the present invention, an image forming method is provided which includes forming an electrostatic latent image on the photoreceptor mentioned above; developing the latent image with a developer including a toner to form a toner image on the photoreceptor; transferring the toner image onto a receiving material; and controlling the surface roughness of the photoreceptor such that the maximum height of the surface of the photoreceptor is not greater than 3 xcexcm in a part of a profile of the surface of the photoreceptor, in a sampling range of 50 xcexcm in a direction perpendicular to the rotating direction of the photoreceptor, and/or the surface of the photoreceptor has a friction coefficient of from 0.3 to 0.5.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.